1. Field of the Invention
The present invention relates to an exposure apparatus which exposes a substrate to radiant energy, and a device manufacturing method using the same.
2. Description of the Related Art
Generally, a semiconductor device is manufactured by performing an exposure process a plurality of number of times for a wafer in the process of manufacturing a semiconductor device. It is therefore necessary to align (overlay) the wafer with an existing circuit pattern in an exposure process of the second and subsequent times. Along with an increase in the packing density of circuits, the recent circuit pattern is required to be fine on the order of nanometers. To meet this demand, alignment between the reticle and the wafer is very important and must be performed with high precision.
The misalignment components between the reticle and the wafer, which are measured in alignment, include:
Wafer Shift
This represents an error in which the wafer shifts in the X or Y direction (FIG. 2).
Wafer Rotation Error
This represents the degree of rotation of the wafer about its center (FIG. 3).
Wafer Magnification Error
This represents an error due to expansion/contraction of the wafer (FIG. 4).
Wafer Orthogonality
This represents an error in which the angle between the X- and Y-axes on the wafer deviates from 90° (FIG. 5).
Conventionally, if a plurality of wafers are to undergo an exposure process, misalignment amount measurement and correction value calculation are performed for each wafer in order to determine correction values for alignment. For precise alignment, it is necessary to calculate correction values by increasing the number of measurement points on the wafer. However, an increase in the number of times of measurement decreases the throughput.
Japanese Patent No. 3451607 determines correction values for wafers in the same lot in the following way. A large number of points in each sample shot on at least one wafer Wk−i of the (k−1)th and preceding wafers are accurately measured and calculated using two alignment sensors, and the measurement and calculation results are saved. Only one point in each sample shot on each of the kth and succeeding wafers is measured using one alignment sensor. Correction values are calculated based on both the measurement results of the wafer Wk−i and the kth and succeeding wafers, thereby ensuring high-precision alignment.
Japanese Patent Laid-Open No. 2001-345243 measures the positions of all shots for each of wafers preceding the nth wafer, separates the misalignment components into nonlinear components and linear components, and holds the misalignment components corresponding to the nonlinear components. For each of the nth and succeeding wafers, linear components are calculated and used, and non-linear components held in advance are used. With this operation, an improvement in the throughput and high-precision alignment are ensured.
If all wafers in the lot have equal misalignment components, a method of decreasing the number of measurement points on the kth wafer by using correction values for the (k−1)th and preceding wafers in exposing the kth wafer, as mentioned above, contributes to an improvement in the throughput. When all wafers in the lot have undergone an exposure process in the same manufacturing line, they are expected to have equal misalignment components.
However, if a plurality of manufacturing lines are used for one wafer in accordance with a difference in required exposure precision in order to improve the overall throughput in the factory, wafers from different manufacturing lines may mix in one lot. If not all wafers in the lot have equal misalignment components, existing correction values cannot be used suitably. When the existing correction values are adopted despite this, a defective product may be manufactured. Eventually, correction values must be calculated by setting a large number of measurement points for all wafers.